The invention relates to a method and an apparatus for improving the surface quality by an in-line surface treatment of slabs which are produced in a continuous process from strip casting up to hot rolling, for example according to the Compact Strip Production (CSP) process with a heating furnace arranged between the casting machine and the hot rolling train. With the help of an oxidation device containing a nozzle or nozzle-like devices, an oxidizing flame or an oxidizing gas mixture is applied to the upper and lower sides of the slab and is targeted so that a defined outer surface layer of the slab with a layer thickness at each slab side up to 4 mm is oxidized; the oxidation product on the slab surface can be solid or liquid.
A related method is disclosed in U.S. Pat. No. 2,453,019 A. A different solution is shown in U.S. Pat. No. 2,312,418 A.
In strip casting plants, particularly in slab casting plants such as for example CSP-plants, cast strands are produced dimensions approaching the end product. Since slabs are removed from the end product with only a few process steps, a smooth and flawless upper surface of the slab is necessary.
Thin and thick slabs produced by continuous strip casting up to 400 mm thick can have near and on the outer surface small imperfections and casting powder residue can be present along with changes in the chemical composition in the edge regions of the outer surface.
Imperfections are for example oscillation marks which because of process technical grounds are unavoidable, and, for example pores. Chemical changes in the steel composition in the edge regions of the outer surface can be caused by material exchange between the steel and the casting powder based on diffusion processes at high temperatures in the mold. Smaller imperfections, casting powder residues and changes in the chemical composition near the outer surface are mostly removed during the casting and heating processes and by rolling of the slabs. In unfavorable situations however, cosmetic surface imperfections are visible in the pickled hot strand, i.e. in the form of oscillation mark shadows.
EP 0 867 239 B1 describes a method for producing hot wide strand, wherein the cast slab strand is divided in sections and, after a tempering treatment in a continuous oven, is rolled out. In order to improve the quality of the outer surface of the slab sections before rolling, it is proposed to interrupt a tempering process of a defective slab section by an outer surface treatment by grinding or flame-scarfing in an outer surface processing device and to carry out the outer surface treatment at a variable speed that is decoupled from the casting speed. If necessary, the slab sections are conducted to a segment of the continuous furnace that is laterally adjacent to the finishing line for carrying out the outer surface treatment.
A method and an apparatus for improving the outer surface of a cast strand of a preferably slab casting plant, by an in-line removal of the outer surface of the cast strand are known from EP 1 093 866 A2. Removal is carried out after the cast strand passes through a heating oven shortly before the in-line rolling of the cast strand. For this purpose electric arcs, laser beams or mechanical means are used, preferably however hot flames with an in-line flame machine with our without mechanical working of the surface of the cast strand. In order to avoid or substantially avoid subsequent oxidation or secondary scale formation on the treated outer surface, it is beneficial to hold the casting strand under inert gas or flue gas or to directly roll after the hot flaming.